Once-through vapor generator variable pressure start-up system



Nov. 22, 1966 w. D. STEVENS 3,286,465

ONCE-THROUGH VAPOR GENERATOR VARIABLE PRESSURE START-UP SYSTEM Filed April 24, 1964 2 Sheets-Sheet l ATTORNEY Nov. 22, 1966 w. D. STEVENS 3,286,466

ONCE-THROUGH VAPOR GENERATOR VARIABLE PRESSURE START-UP SYSTEM Filed April 24, 1964 2 Sheets-Sheet 2 \/l H/H PRESSURE rake/NE \j v) 04 l b 3500 a sooo V Lu zsoo 0 STA/7- l/P BY-PASS SYSTEM 2000 TMm/0 OPE/a4 r/o/v. u lsoo Y i |000 *s 50o l TH/eornf PRESSURE Q SEPA/QA r/ N ,OR s URE. o lo 203040 506070 o9o|oo 10b-@CENT OAD ATTORNEY United States Patent O 3,286,466 ON CE-THRUUGH VAPOR GENERATOR VARIABLE PRESSURE START-UP SYSTEM William D. Stevens, North Caldwell, N J., assignor to Foster Wheeler Corporation, New York, NX., a corporation of New York Filed Apr. 24, 1964, Ser. No. 362,229 13 Claims. (Cl. 60-105) This invention relates to improvements in a start-up and low load system for vapor generators, and in particular to apparatus, methods, and controls for starting-up, re-starting and low load operation of a subcritical or supercritical vapor generator of the forced flow once-through type.

An important item, in the operation of a once-through vapor generator is the by-pass system around the turbines. This system is necessary to protect the furnace walls by providing suilicient circulation of uid at start-up and low loads. However, the quantity of water or steam flowing through the tubes to prevent the tubes from overheating and burning out may exceed the turbine capabilities, and the by-pass system then passes the excess flow around the turbine.

Protection of the high pressure turbine parts also is an important matter in the design of a once-through generator. These parts, which must be capable of handling flows at high temperatures and at very high pressures, for instance 3500 p.s.i. constitute a large portion of the expense of a turbine unit. Avoiding excessive thermal stresses in the parts is, therefore, a critical consideration.

It is known in by-pass systems to use a dash drum in a turbine by-pass line for the purpose of handling the excess flow and separating the vapor content from the flow for warming and rolling of the turbine. It is also known in start-up systems to use a throttling valve upstream of the turbine for the purpose of furnishing reduced pressure vapor to the turbine. This low pressure throttle vapor allows start-up operation of the turbine with minimum temperature differentials (and therefore minimum thermal stress) in the turbine inlet parts.

The objectives in such prior systems include the avoidance of excessive thermal losses, the reduction of the time required for start-up, and the exercise of care in the protection of turbine parts and vapor generating and superheating sections against thermal shock and high ternperature damage. It is found that the application of principles in accordance with the present invention results in substantial improvements in these respects beyond those heretofore achieved.

Accordingly, it is an object of the invention to provide a system in which, during start-up, or low load operation, the heating surfaces are more fully protected, and in which steam is made available earlier in the start-up period for warming up .and rolling the turbine, and for other uses; to reduce to a minimum the period required for start-up; and, incidental to this, to reduce the heat losses which result from start-up and low load operation. It is also an object of the invention to supply the turbine with steam at a pressure and temperature which is gradually increased during the start-up period and which reaches full throttle pressure at a load compatible with turbine design for maximum protection of turbine parts.

These and other objects are accomplished in accordance with the invention by providing in a once-through unit; which includes in series a vapor generating section, primary and secondary or finishing superheating sections, a condenser, and a condensate feed system; a first by-pass loop comprising a first by-pass conduit from the exit of the primary superheater leading to a vapor-liquid separator (which may be a flash tank), sub-loop conduits lead- 3,286,466 Patented Nov. 22, 1966 ICC ing from the separator to points of use which include the nlshing superheater, the condenser and condensate feed system, a stop valve in the by-pass conduit ot open and close the by-pass loop, pressure reducing valves in said sub-loop conduits to maintain a desired pressure in the furnace and primary superheater circuitry; a second bypass loop comprising a second by-pass cond-uit also leading from the exit 0f the primary superheater, a pressure reducing valve in said second conduit, and means communicating the second conduit with at least one of said sub-loop conduits.

For convenience, the second by-pass conduit may lead to the separator from which the flow is distributed to the sub-loop conduits.

In operation, during a cold start, the iluid required for cooling the circuitry flows through the rst by-pass loop. When heating of the unit reaches a point where the flow is vaporized, the second by-pass loop is utilized to load the turbine an amount comparable to the percentage of full load flow made available. Further loading up to full load is achieved by increasing firing rate and pumping rate in the unit.

It is a concept of the invention that the pressure reducing valves in the various sub-loops will maintain in the separator and upstream circuitry a low pressure. This permits vapor to be flashed in the separator earlier in the starting-up cycle for warming and rolling the turbine and for other uses. It also means that heat is imparted to the circulating fluid at a lower fluid pressure for better heat transfer. This contributes towards reducing start-up time and obtaining minimum loss of start-up heat input.

The second by-pass loop is used at low loads, or in a hot re-start, when the minimum ilow vaporized in the heating sections exceeds turbine demand, excess vapor flow being disposed of through the second loop. The pressure reducing valve in the second loop maintains desired pressure in the circuitry.

Since at least a portion of the vapor during start-up may pass to the finishing superheating section downstream of the separator, this portion is supplied to the turbine in a superheated state. In this way, the turbine gets a superheated vapor earlier in the start-up period. A third by-pass loop leading from the exit of the finishing superheater permits the vapor flow to be supplied to the turbine at a heat level required for maximum protection ofthe turbine parts.

The invention and objects and advantages thereof will become apparent upon consideration of the specification and accompanying drawings, in which:

FIGURE 1 illustrates schematically an embodiment of a start-up system in accordance with the invention;

FIGURE 2 illustrates a throttle valve and governor valve control arrangement for a high pressure turbine;

FIGURE 3 is a diagram showing operation of a turbinegenerator unit in accordance with the invention during a cold start with respect to variation in pressure with percent load at the turbine throttle;

FIGURE 4 is a diagram for the unit of FIG. 3, showing the variation of the turbine governor valve position with load.

Referring to the embodiment of FIG. l, there is illustrated schematically a vapor generating and turbine installation which includes in series an economizer 12, furnace passes 14, and roof and convection enclosure passes 16. Also constituting sections of the installation are a primary and/ or platen superheating section 22 and a finishing superheating section 24. During normal operation of the unit, the flow is through the superheating sections and from lthe outlet 24a of the n-ishing superheating section t-o a high pressure turbine 26, the exhaust steam from the turbine being reheated at 28 and passed to a low pressure turbine 30, and from there to a condenser 32. Being of the once-through type, the system is pressurized by a feed pump 34, the feed flowing from the condenser through low pressure heaters 36, deaerator 38, storage tank 40, high pressure heater 42, and separating heater vessel 44 to the economizer 12. The heater vessel 44 is described in U.S. patent application Serial No. 337,860, filed January 15, 1964 by Robert L. Lytle, George P. Moran and William D. Stevens. For purposes of this application the section 46 of the vessel 44 contains heat exchanger elements of the type used in feedwater heater 42, for instance as described in U.S. Patent No. 2,946,570, John W. West. Also, for the purposes of this application, the term condensate feed system shall be deemed to 'comprise the low pressure and hi'gh pressure heaters, including the heat exchange portion 46 of the separating heater 44, the deaerator, the storage tank, and the feed pump.

In accordance with the invention, the turbine by-pass and start-up system includes a first by-pass loop comprising a vapor-liquid separator 48 disposed in a first bypass line 50 leading from the outlet 22a of the primary superheater 22 and by-passing a main flow line valve (or valves) 52 between the superheating sections 22 and 24. A stop valve 54 in the first by-pass line 50 separates the separator from the outlet 22a of the primary superheating section 22. Also disposed in the first by-pass line 50 is a spray attemperator 56. In the embodiment shown, the separator 48 constitutes the upper portion of the separating heater 44 as described in U.S. application Serial No. 337,860.

The piping and valves for the distribution of the bypass vapor flow from the separator 48 consist of a vapor line 60 extending to the inlet end 24b of the finishing superheater 24, this line including a pressure reducing valve S8, and vapor flow branches 60a and 60b to other points of use including the turbine Igland seal regulator 62 and the deaerator 38, these lines being opened and closed by pressure reducing valves 64 and 66. In addition, vapor from the separator 48 can flow to the heater 46 via internal line 44a in the vessel 44, the exhaust flow from heater 46 cascading to high pressure heater 42 via exhaust line 60C. A pressure reducing valve 67 responsive to pressure in vessel 44 or other means may be used to maintain pressure in the furnace and by-pass circuitry. Drain or liquid flow from the separator 48 is handled by a line 68 having branches 68a and 68b leading to the condenser hot-well 70 and to the high .pressure heater 42, respectively, these lines being opened and closed by relatively small pressure reducing valves 72 and 74.

The by-pass system also includes a second loop comprising a conduit 75, containing pressure reducing valve 77, leading from the outlet 22a of the primary superheater to the sub-loop lines 60a and 60b. The conduit 75 may be in fluid communication with the sub-loops through a suitable header 76, or for convenience may lead to the separator 48 from which the flow can be distributed to the sub-loops 60a and 60b. During use of the conduit 75, the valves 54 and 58 in the first loop between the separator and superheating sections are closed.

Also constituting part of the by-pass system is a third loop including conduit 78 leading from the outlet 24a of the finishing superheater 24 to the condenser, bypassing the high pressure and low pressure turbines, a pressure reducing valve 79 in the line, and a spray at- -temperator 80 immediately upstream of the entrance port of the condenser. The conduit 78 is joined by a branch line 60d from the header 76 through valve 82.

The invention will be better understood by way of example with reference to FIGS. 1, 3 and 4, showing steps of operation of a once-through unit in accordance with the invention, and FIGS. 2-4 showing the manner of tie-in of the unit with a turbine. Although in the following example, specific numbers are given with respect to pressures, ow rates, temperatures and the like, it is to be understood that the concepts of the invention are not limited to these specifics.

INITIAL HEATING OF THE ONCE-THROUGH UNIT Initially, in the cold start-up period, the feed pump 34 is driven to pressurize the unit, and the fluid flow necessary for cooling the circuitry to the outlet 22a of the primary or platen superheater 22 is established. Since the turbine is incapable of handling this flow, the output from the superheater is fed through stop valve 54 to the separator 48 (of vessel 44), and through drain line 68 to the condenser hot well (line 68a). In this example, a ilow rate of 30% of full load flow through the unit is required for satisfactory cooling of the high pressure circuitary. The pressure in the separator is set for 1500 p.s.i., a little less than half of full load pressure and it is maintained at this pressure by means of the valves 72 and 74 in the sub-loop lines 68a and 68b. The furnace and superheater circuitry pressure follows separator pressure.

The high pressure heater stages drain as shown, through line 84 to the deaerator storage tank 40. The condensate from the condenser 32 is pumped by the condensate pump 85 through the low pressure heaters 36 to the deaerator storage tank 40.

At this stage of start-up, the main flow line valve 52, between the outlet end of the primary or platen superheater 22 and inlet end 24b of the finishing superheater 24 is closed so that no flow passes through this valve. Also closed are the separator vapor discharge line valve 58 leading to the inlet end 24b of the finishing super-v heater and the turbine by-pass line valve 79, between the outlet end 24a of the finishing superheater and the condenser 32. Also at this stage of start-up, the vapor lines 60a, b and c from the separator tank are closed by valves 64, 66 and 67.

The burners are put into operation, for instance, at about 15% full load firing rate, controlled so that the furnace exit temperature entering the area of the finishing superheater does not exceed 1200 F., the maximum safe temperature for the tube metal. At this point, the flow remaining at 30% of full load flow, no significant steam quantity is flashed in the separator 48, but as heating is continued, steam flashing will take place in the separator in increasing quantities and flashed steam is then supplied via line 60 to the sub-loops 60a and 60h and via passageway 44a to high pressure heaters 46 and 42.

As the separator pressure reaches its set point value of about 1500 p.s.i., at which it is controlled for the full start-up cycle, the pressure reducing valve 58, in the vapor line 60 leading from the separator vapor space to the inlet of the finishing superheater 24, is cracked open on remote manual control (RM) to furnish Warming steam to the main steam line. Also, the valve 79 in the turbine by-pass line 78 is cracked open on remote manual control (L) to hold the desired separator liquid level (item condenser 32. As an alternative, any suitable main steam line drain may be provided for this purpose.

All of the separator drain flow is still routed through the valves 72 and 74 to the condenser hot well and high pressure heated. Valves 72 and 74 respond to level conl320; (L) to hold the desired separator liquid level (Item INITIAL TURBINE ROLLING When 5% of full load steam ow becomes available from the separator 48, a portion is diverted through valve 58 and the finishing superheater for rolling and warming the high pressure turbine, the remainder being disposed of through the vapor lines 60a and 60b leading from the vapor space of the separator to the various sub-loops.

A typical high pressure steam turbine unit (FIG. 2) will include a main throttle stop valve 86, and a plurality of governor control valves 88 in series with the stop valve. Associated with the stop valve 86 is a relatively small by-pass control valve 90 for control of the flow at low loads.

The Wa-rming and rolling flow from the outlet of the separator to the high pressure turbine through the finishing superheating section is superheated slightly in the finishing superheated. At this stage, the turbine throttle stop valve 86 (FIG. 2) is closed and the by-pass valve 90 controls t-he flow to the turbine, reducing the pressure of the flow to a desired pressure entering the turbine.

Usually, only about 2 to 3% How is needed for the purpose, of warming and rolling the turbine, but if more than 2 to 3% flow is needed through the finishing superheater for control of the enthalpy of the fluid at the turbine by-pass, the remainder may be dumped through the turbine by-pass line 78, from the outlet 24a of the finishing superheater, to the condenser 32. The spray attemperator 80, on temperature control, insures that the temperature of the fluid entering the condenser does not eX- ceed design limits of the condenser.

During the final stages of heating of the generator with separator 48 pressure held at 1500 p.s.i. and prior to loading, the initial firing rate is maintained. Furnace exit temperature still is monitored and should not exceed 1200 F., and the 2 to 5% rolling flow for warming the turbine is maintained from the separator to the finishing superheater and the high pressure turbine.

TURBINE SYNCHRONIZATION AND LOADING Generally speaking, load assumed by the turbine varies linearly with the amount of steam fed to the turbine. As the heating is continued in the unit, the steam temperature entering the turbine first stage through the turbine stop valve by-pass 90 (FIG. 2) increases; the turbine parts approach a state of equilibrium; and at this point the turbine is synchronized and loaded. This is accomplished by regulating separator vapor discharge valve 58 (the pressure reducing valve at the inlet end 24b of the nishing superheater 24) through set point adjustment of the turbine throttle pressure controller (TPC), FIG. 1 and signal 108. The separator pressure is at the set point value of 1500 p.s.i. and opening of valve 58 increases the turbine load to by increasing turbine throttle pressure to 1500 p.s.i. (The valve 58 is sized to permit about 15 of full load steam flow therethrough.) This is shown as point R in FIG. 3, which illustrates the variation of turbine load with throttle pressure in a turbine unit in accordance with the invention.

When the entire 30% start-up flow in the unit is vaporized, the main flow line valve 52 between the superheating sections is-opened in the ull open position and valve 58, between the vapor space of the separator and the inlet end of the kiinishing superheater 24 is closed. In response to a turbine load signal (136), the pressure reducing valve 77 in the by-pass line 75 (the second by-pass loop) is placed into service and the stop valve 54 of the tirst loop is taken out of service, so that a portion of the vapor generated passes directly to the various vapor sub-loops, for instance 60a, 60h, and a portion through main valve 52 to the turbine. `The amount of ow diverted through valve 77 depends upon the turbine load signal and is commensurated with the load demand at this time.

The load is increased further by increasing the throttle pressure through set point adjustment of the throttle pressure conroller (TPC). This signal (108) increases furnace pressure and throttle pressure by throttling the bypass pressure reducing valve 77 through controller 110. Downstream pressure control of valves 77 and 82 is a limiting control in case of sub-loop control failure. Firing rate will automatically adjust to provide desired throttle steam conditions.

When 30% load is achieved, i.e., the full start-up flow passes to the turbine, the primary superheater outlet pressure is 3000 p.s.i., point V (in FIG. 3,) and pressure reducing valve 77 is entirely closed, removing the second start-up loop from service. Loading the turbine from 30% to 100% load is accomplished by turbine governor valve controls and by combustion control regulation of throttle pressure and temperature (to be described). This is represented by lines V-T-U on FIGS. 3 and 4. The combustion control regulation includes means to control the feed pump output and to meter the fuel and ai-r supply, so that throttle pressure andtemperature are maintained.

Some advantages of the invention with respect to a cold start should now be apparent. Particularly evident should be the advantage of initially improved heat transfer at lower pressure (1500 p.s.i.) in the vapor generating and superheating sections to reduce the start-up period, and correspondingly, heat losses from start-up. At the same time, optimum conditions of pressure and ilow are maintained in the furnace circuitry for protection of the heating surfaces.

A rfurther advantage is the flexibility offered by the system. The first by-pass loop permits separation of vapor and liquid and transmission of vapor to points of use earlier in the start-up. The second lay-pass loop with pressure reducing valve 77 sized to handle `from 0-30% of the flow facilitates, once the start-up ow is entirely evaporated, increasing the flow and pressure of the vapor to the turbine and bringing the load of the turbine up. In this respect the rate of increase of turbine load can easily be adjusted to avoid undue stress in the turbine.

HOT RE-START The invention is also useful for hot re-starts. A hot re-start is initiated by establishing a 30% fluid ow through the once-through unit and platen and/ or primary superheater outlet. If at the hot re-start, the uid is in a vapor state and is at a pressure above maximum design pressure for the separator 48, in this instance 1500 p.s.i., a portion of the fiuid flow from the outlet of the primary and/ or platen superheater .is through the pressure reducing valve 77 (this valve can handle 0-30% full load vapor flow) and the second by-pass loop to the sub-loops. Inthis case, valves 52 and 58 are closed, and the turbine is brought up to 30% load by set point adjustment of throttle pressure controller (TPC).

If at the hot re-.start thefluid pressure is below 1500 p.s.i., the fluid flow from the outlet of the primary and/ or platen superheater is to the separator through the stop valve S4. The separator drain flow is routed through valve 72 to the condenser hot-well, and the separator steam flow is routed to the sub-loops including the gland seal regulator through valve 64, the deaerator through -valve 66, the high pressure heater 46 through conduit 44a, and to the condenser through line 60d and valve 82.

The burners are set at start-up firing rate (for instance 15% full load firing rate). The turbine by-pass valve 79 of the third by-pass loop between the outlet of the finishing superheater and the condenser is opened to pass the flow through the finishing superheater to the condenser. Once established, turbine by-pass valve 79 is closed and synchronizing and loading of the turbine is accomplished immediately increasing firing rate to hold throttle steam conditions at a predetermined value. From this point on, aspects of the cold start are repeated to full load.

The simplicity and flexibility of the system for a hot re-start should be evident.

Depending on the degree to which the unit has cooled, the first and second by-pass loops upstream of the finishing superheater permit the operator to pass a predetermined amount Iof vapor to the finishing superheater where the throttle stream is effectively -reheated prior to admission to the turbine. These loops in combination with the third by-pass loop enable the operator to better match turbine first stage inlet steam temperature with the temperature of the turbine inlet parts. If full throttling is undertaken through the turbine throttle valve, the temperature of the incoming fluid would fall below that of the turbine inlet parts, or at least it would make it difficult to match the temperature with turbine inlet parts. By obtaining the exact enthalpy required at the outlet of the finishing superheater, the temperature of the fiuid can be reduced to that of the turbine parts by the turbine throttle valve by-pass (90, FIG. 2).

In addition, it is evident that the start-up can easily and efficiently be effected from any heat level or enthalpy level of the circulating fluid. For a quick start, a minimum degree of manual control is required, and the load is smoothly applied to the turbine for maximum protection of turbine parts.

LOW LOAD OPERATION The invention is also useful for loW load control. A minimum flow, for instance a 30% ow, is required in the high pressure furnace circuitry for cooling the circuitry. In reducing the turbine load, through turbine governor valve control of the flow, no problem is experienced as the flow is decreased to this minimum iiow by combustion control regulation of the feed pump and firing rate. The flow is in the main flow path valve 52, FIG. 1, and full throttle pressure is maintained at the turbine inlet.

Below 30% flow to the turbine, the start-up system .is advantageously used to dispose of the excess ow at the primary superheater outlet. However, for this purpose, the pressure reducing valve 77 in line 75 handles the ow, holding full pressure at the turbine throttle. The set point for the reducing valve 77 responds to the turbine first stage outlet pressure load signal represented by dotted line 136 leading from the turbine to control box 110 for valve 77. If the capacity of the reducing valve 77 is exceeded, i.e., more flow must be diverted than the valve is capable of handling, the additional flow is passed through valve 79 to the condenser. This valve also is under set point regulation in response to the pressure (pressure controller 112) at the finishing superheater outlet.

At very low loads, or on sudden changes in load, the fluid temperature at the primary superheater outlet may be too high, and can be reduced by means of spray attemperators 114 in response to an anticipatory turbine load signal (first stage turbine outlet pressure) and to the main steam temperature set point as compared to the measured temperature (items 136 and 132 respectively).

THE CONTROLS Controls for the generator-turbine unit have been mentioned in the specification in -greater or lesser detail.

In the cold start-up sequence, the feed pump (34, FIG. 1) is on flow control to hold the minimum flow (for instance, 30% of full load fiow). This is in response to a signal (120) from flow orifice 122 (at the inlet of the economizer 12) acting through controller 123 and signal (124). The separator sub-loop valves and other start-up system valves are set as described in the foregoing description on the cold start-up sequence, holding the desired flash tank pressure. With main fiow line stop valve 52 in service, the furnace pressure will follow separator pressure set point. The metered fuel and air control (item 126) for the burners (the fuel and air input) is initially on manual adjustment (controller 127 and signal 129).

When turbine rolling and warming steam is available from the separator, the desired steam temperature for the high pressure turbine is achieved by manual adjustment of the firing rate and control of the ow through the finishing superheater (excess flow passing through the turbine by-pass valve 79). It will be recalled that the throttle stop valve by-pass (90, FIG. 2) admits a 2-3% flow to the turbine first stage. Manual control (RM) of the turbine by-pass valve (79, FIG. 1) passes a greater or lesser flow through the finishing superheater for control of the temperature at the finishing superheater outlet.

Up to loading the turbine, the temperature of the uid entering the turbine is gradually increased, the fuel and air input to the burners being subject to minor manual adjustment from the initially set firing rate.

As the turbine parts reach a state of equilibrium, the turbine is synchronized and loaded up to, for example, 15% of full load. This is accomplished by closing the turbine by-pass valve 79, and the full ow through the finishing superheater, for instance a 15% full flow, is fed to the turbine. Since the by-pass control (valve 79) is no longer used for temperature control, the temperature of the finishing superheater outlet is controlled and held at a set point by means of set point adjustment of a temperature controller (130, FIG. 1) to which is fed a temperature signal (132). The signal (142) from the controller is compared in controller 140 to an anticipatory load signal (139), the latter controlling emergency spray attemperators 114 located immediately upstream of the finishing superheater. It will be recalled that of the 30% start-up flow, about 15% only, at this stage, is passed through the finishing superheater to the turbine, the remainder being disposed of in the separator by-pass system.

During this period of loading to 15%, it is envisioned that although the fuel firing rate and air input responds primarily to manual control (127), it will adjust for feed and main steam temperature to a limited extent. For instance, a main steam temperature (signal 132) below the set point will increase the firing rate (signal 142), whereas above the set point, with full attemperation, it will decrease the firing rate. The feed water temperatuer signal (144b) also is compared to the main steam temperature (signal 142). This combined signal then modifies the manual control signal (127-129) for control of the fuel and air supply. The purpose of the feed water temperature signal is, that when the by-pass system is used, with fiow to the high pressure heaters (for conserving heat), the feed water takes an initial jump in temperature. This must be compensated for by reducing the fuel and air input.

At a 15% load and above, control of the fuel and air input can be changed from manual to automatic by providing, for automatic control, a load demand signal (138), for instance a 15 demand signal, which represents the load applied to the turbine. This signal is compared in controller 133 with turbine first stage outlet pressure I(136) and throttle pressure (137 to provide a load signal (139). The load signal (139) is then modified in a true control sense with the main steam temperature signal (142) and feed temperature signal (114b) as described above, the resultant thereof controlling the fuel and air input.

Simultaneously with the increase in fuel and air input to obtain a load increase above 15%, the turbine throttle pressure is varied linearly or is characterized with load (as shown in FIG. 3), by the first stage outlet pressure or load signal (137) transmitted to controller 110. Set point adjustment of the turbine throttle pressure controller (TPC) provides signal (108) to controller 110 by which the pressure increase matches load increase. This was described in some detail in the above discussion of a cold start.

Following removal of the start-up system from service, the feed pump 34 also responds to the load demand signal (13911) and the heat input, ow rate, and pressure increase, is programmed in a suitable manner with the load signal (139) and temperature signal (142).

For low load operation of the turbine, at constant throttle pressure, the flow is initially reduced (down to the minimum 30% flow) by combustion control regulation of pumping and firing rate (load demand signal 138-139, 139b to the feed pump 34 and to metered fuel and air system 126). At and below 30% flow, the turbine first stage load measuring signal (136), fed to control 110 for valve 77, will permit this valve to respond to a con- 9 stant set point pressure control (TPC), so that as the turbine load decreases, increased flow is by-passed, with full throttle pressure maintained in the circuitry. Other aspects of low load control should be apparent from the earlier description on low load operation, and the above description covering control of the fuel and air input or firing rater Although the invention has been described with respect to specific embodiments, many variations within the spirit and scope of the invention as defined in the following claims will be apparent to those skilled in the art. For instance, the points4 of use of by-pass vapor and liquid and means for transmittal thereof can be widely varied.

What is claimed is:

1. A -once-through Vapor generator comprising a main flow path includin-g a plurality of heating surfaces and a condensate system in series;

a start-up system including first and second by-pass loops leading from a first point intermediate s-aid heating surfaces to the condensate system;

the first by-pass loop including separating means for separating a mixed phase ow into a vapor stream and a liquid stream, a vapor line to return the vapor stream to a second point intermediate the heating surfaces but downstream of the first point;

a shut-off valve in the main ow path intermediate the first and second points;

the second by-pass loop including a conduit, a pressure reducing valve in the conduit adapted to maintain pressure in the heating surfaces Aand to divert a variable flow to said condensate system.

2. A generator according to claim 1 including at least one sub-loop line in the first by-pass loop leading from the `separating means to the condensate system, a pressure reducing valve in said sub-loop line adapted to maintain pressure in the heating surfaces.

3. A generator according to claim 1 wherein the vapor Iline of the first by-pass loop is sized to handle only ya fraction of the minimum flow required for cooling the heating surfaces, until the flow in the heating surfaces upstream of said loop is entirely vaporized, the second by-pass loop being sized to handle further increase in flow until full pressure is achieved in the generator.

4. A generator according to claim 1 wherein the generator includes primary and secondary superheating sections, the first and second points being intermediate said sections.

5. A generator according to claim 1 in combination with a turbine wherein said heating surf-aces have an outlet end in flow communication with the turbine, the generator including a third by-pass loop leading from said outlet end to the condensate system, the third by-pass loop permitting a greater by-pass of flow from the heating surfaces outlet end th-an the turbine can handle so that the enthalpy of the fluid at the outlet end required by the turbine can be obtained.

6. A once-through vapor -generator comprising a main flow path including a plurality of heating surfaces and la condensate system in series;

a start-up system including first and second by-pass loops leading from a first point intermediate the heating surface-s;

the condensate system comprising points of use including heat exchange elements for the by-pass flow;

the first by-pass loop including separating means for separating a mixed phase flow into vapor and a liquid, a vapor line to return at least a portion of the vapor to a second point intermediate the heating surfaces but downstream of the first point, subloo-p lines to separately transmit a portion of the Vapor and the liquid to said points of use, pressure reducing valves in said sub-loop lines, a pressure reducing valve in said vapor line;

a `shut-off valve in the main flow path between said first and second points;

the second by-pass loop including a conduit, a pressure reducing valve in the conduit, the conduit in fluid communication with at least one vapor subloop line;

the first by-pa-ss loop vapor line :being sized to handle only a fraction of the minimum flow required for cooling the heating surfaces until the flow in the heating surfaces upstream of the first loop is entirely vaporized, the second by-pass loop being sized to handle further increase in flow until full pressure is` v achieved in the generator.

7. A vapor generator according to claim 6.wherein said'heating surfaces include vapor generating and primary and secondary vapor superheating sections, said first and second points intermediate the heating surfaces being between the primary and secondary vapor superheating sections.

8. A vapor generator according to claim 6 wherein one of said points of use is a feed-water heater in which the vapor iiow from the separating means is passed in indirect heat exchange with feedwater to the heating surfaces.

9. A vapor generator according to claim 6 including a header arranged to feed a vapor flow to 'at least one of said points of use, the first by-pass loop vapor line and second by-pass loop conduit both arranged to transmit a vapor fiow to the header.

10. A vapor generator according to claim 6, the condensate system including a condenser, at least one condenser sub-loop line leading to the condenser, means communicating both the first by-pass loop vapor line and second by-pass loop conduit with the condenser sub-loop line, a pressure reducing valve in said condenser sub-loop line.

11. A vapor generator according to claim 10 including a third by-pass conduit, the generator heating surfaces having an outlet end, the third conduit leading from said outlet end to the condenser, -a pressure reducing valve in the third conduit.

12. A vapor generator according to claim 6 including a control arrangement adapted for turbine utilization of the vapor generated, the heating surfaces having an outlet end immediately upstream of the turbine, comprising a controller responsive to pressure at the outlet end;

means for set point adjustment of the controller;

the controller being arranged to open and close the second by-pass loop pressure reducing valve depending `on said set point adjustment thereby varying the pressure in the heating surfaces;

means for supplying a load demand signal to increase `fuel and air input to the generator;

first means for modifying the load demand signal responsive to the pressure at the heating surfaces outlet end relative the load;

an-d second means for modifying the load demand signal responsive to vapor temperature at the heating surfaces outlet end relative feed temperature.

13. A once-through vapor generator comprising a main flow path including in series vapor generating sections, primary and secondary superheatin-g sections, a condensate system;

a start-up system including first and second by-pass loops leading from the outlet end of the primary superheating section to the condensate system, the latter including points of use for by-pass vapor and liquid flow;

the first by-pass loop including separating means for separating `a mixed phase flow into a vapor stream and a liquid stream, a vapor line from the separating means to return the vapor -stream to the inlet end of the secondary heating section downstream of the primary superheating section outlet end, subloop lines from the separating means to convey separately vapor and liquid to the points of use in the condensate system;

pressure reducing means in the sub-loop lines and vapor line for maintaining pressure in the vapor lgenerating sections and primary superheating section;

a shut-off valve in the main ow path intermediate primary superheating section outlet end and secondary superheating section inlet end;

the second by-pass loop including a conduit extending between the primary superheating section outlet end and -at least one of the vapor sub-loop lines;

a pressure reducing valve in the conduit, the conduit and valve 'being adapted to divert a variable vapor ow to the condensate system.

References Cited by the Examiner UNITED STATES PATENTS Schwarz 122-406 Glahe 122-406 Strohmeyer 122-406 X Strohmeyer 60-105 Grabowski 60-105 X Gerber et al. 122-406 Strohmeyer 60-106 MARTIN P. SCHWADRON, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

UNITED STATES PATENT oEEIcE CERTIFICATE 0F CORRECTION Patent No. 3,286,466 November 22, 1966 William D. Stevens It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 3, for "ot" read to Column 3, line 13, for "John W. West" read John M. West column 4, line 59, for "(L) to hold the desired separator liquid level (item" read (RM) to allow the warming steam to pass to the line 64, for "heated" read heater Column 5, line 8, for "superheated" read superheater Signed and sealed this 12th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A ONCE-THROUGH VAPOR GENERATOR COMPRISING A MAIN FLOW PATH INCLUDING A PLURALITY OF HEATING SURFACES AND A CONDENSATE SYSTEM IN SERIES; A START-UP SYSTEM INCLUDING FIRST AND SECOND BY-PASS LOOPS LEADING FROM A FIRST POINT INTERMEDIATE SAID HEATING SURFACES TO THE CONDENSATE SYSTEM; THE FIRST BY-PASS LOOP INCLUDING SEPARATING MEANS FOR SEPARATING A MIXED PHASE FLOW INTO A VAPOR STREAM AND A LIQUID STREAM, A VAPOR LINE TO RETURN THE VAPOR STREAM TO A SECOND POINT INTERMEDIATE THE HEATING SURFACES BUT DOWNSTREAM OF THE FIRST POINT; A SHUT-OFF VALVE IN THE MAIN FLOW PATH INTERMEDIATE THE FIRST AND SECOND POINTS; THE SECOND BY-PASS LOOP INCLUDING A CONDUIT, A PRESSURE REDUCING VALVE IN THE CONDUIT ADAPTED TO MAINTAIN PRESSURE IN THE HEATING SURFACES AND TO DIVERT A VARIABLE FLOW TO SAID CONDENSATE SYSTEM. 